I tested XnView's ability to read IPTC as written by the two other apps mentioned in the topic starter [along with Photoshop], on a JPEG file that did not have any (only a short string in the Comment header). XnView "saw" the annotations written by Photoshop, of course, and iView's showed up in Properties, and there <i><b>was</b></i> a corner tag in the Browser. Likewise GraphicConverter. I suppose the issue may be with <u>changes</u> made by the latter two apps, and this is something I've yet to test. Starting with a "virgin" file, annotations-wise, is hardly a way to get the full picture of what exactly XnV won't do.
In other parts of the graphics-app universe, GraphicConverter, for the last whole version at least (5.x), has an XML/XMP tab in its Info window. The GiMP allows for viewing the same information in its Properties window. Regardless of the result of my next "test", I think it's time for XnView to catch up. Either that or drop back to just EXIF support.